In order to avoid the problems introduced by iodine incorporation into glucagon, 3H-glucagon was prepared and was used to reexamine the kinetic relationship between binding and action of glucagon. The results showed that the time courses of glucagon binding to its receptors and of activation of adenylate cyclase correlate closely. This study also revealed that, in the presence of GTP, a high affinity state of the receptors exists, in addition to the low affinity state detected using 125I-glucagon in the previous study. These findings provided new insight on the overall organization and regulation of the adenylate cyclase system. This project has been completed and the findings were published. BIBLIOGRAPHIC REFERENCES: Lin, Michael C., Nicosia, Simonetta, Lad, Pramod M. and Rodbell, Martin: Effects of GTP on Binding of 3H-glucagon to receptors in Rat Hepatic Plasma Membranes. J. Biol. Chem. 252: 2790-2792, 1977.